Conveyor systems in which conveyor belts are driven in a helical path with the belt curving edgewise around a series of vertically spaced loops are commonly used to provide a great length of endless conveyor belt in a relatively small space. Such a spiral conveyor system is disclosed for example in U.S. Pat. No. 3,348,659, the content of which is hereby incorporated by reference. In such a system, the belt is frictionally driven by driving elements which slidingly engage the radially inner edge of the belt in the helical loops. The driving elements move faster than the inner edge of the belt and continuously slide past the belt edge to achieve a frictional as opposed to a direct or positive drive. This prior system provides smooth dependable operation within its safe operating parameters, relative to speed, loading and belt width. When such parameters are exceeded, however, the belt is subjected to excessively high tension which can result in excessive wear and fatigue failure of the belt in addition to causing damage to the conveyor structure. Further, when the safe parameters are exceeded, surging of the belt can result which interferes with its smooth operation and causes disturbance of the products being conveyed.
Attempts have been made in certain prior art conveyor systems to employ a positive drive in which the radially inner edge of the belt is directly driven by the continued abutting engagement between the driving elements and the belt as opposed to a sliding frictional engagement. To obtain a positive drive, the driving elements must extend past the outer belt surface into the belt structure to engage interior drive surfaces of the belt. In a helical conveyor system, such engagement is difficult to initiate and maintain smoothly. Necessarily as the belt tangentially approaches and moves into its first helical loop, the pitch of the links along its radially inner edge changes as the curvature of the edge changes. Initial engagement of the drive elements with the driven surfaces along the belt edge while their pitch is changing results in rough belt operation and excessive wear of the affected components. Moreover, after the initial engagement, any significant variation in the pitch of the driven surfaces while traversing the helical loops is disadvantageous. Such pitch changes occur with any significant variation in the length of belt in the helical path. If there is an increase in pitch, driving contact can be lost. The belt can thus migrate backwardly along the loops and become slack in its approach to the first loop. If there is a decrease in the pitch, excessive belt tension results which causes various problems including surging, excessive wear, fatigue failure and other damage to the system. Still further, the invasion of the driving elements past the edge of the belt to contact the driven surfaces can result in damage if there is excessive penetration resulting in extraneous engagement with parts of the belt other than the intended driven surfaces.
With reference to FIG. 1, U.S. Pat. No. 4,852,720, the content of which is hereby incorporated by reference, addresses the above concerns by providing a conveyor system comprising a conveyor belt 11 positively driven by drive cage 212 around a plurality of vertically spaced helical loops. A pair of supplemental positive drives 25 and 26, which are driven at the same speed and synchronized with the drive cage, engage the belt 11 near its entry into and exit from the helical portion of its endless path. Vertical driving bars 45 on the drive cage 21 positively engage the cross rod heads 14 of the belt 11 in its helical path. The driving surfaces of the bars 45 are relieved at 61 and 62 to prevent driving engagement with the rod heads 14 over an inlet section of the first loop and an exit section of the last loop. The driving bars 45 also engage the ends of the rod heads 14 to limit the penetration of the driving bars past the edge of the belt. The driving bars include means which may comprise gates 105 or ramps 121 to permit controlled intermittent slip-backs of the rod heads at predetermined intervals.
While successful, there still exists a need in the marketplace for a conveyor system for maintaining belt orientation, providing improved positive edge drive capability, and permitting smooth engagement of the driving bars with the belt edge as the belt enters the first loop at one end of the helical portion and smooth disengagement of the bars from the belt edge as the belt leaves the last loop at the opposite end of the helical portion.